1. Field of the Invention
This invention relates to an improved high temperature-high pressure electrode. It is directed to apparatus for electrochemical potential measurements in a high temperature-high pressure environment with particular application in electrochemical potential measurements for monitoring corrosion in hostile environments such as nuclear power plants.
2. Background Information
The majority of electrode kinetic studies in high temperature aqueous systems have employed external reference electrodes which are maintained at ambient temperature. External reference electrodes have their electroactive elements at ambient temperature outside the high temperature system. Communication between the electroactive element and the high temperature zone is made with a non-isothermal (cooled) KCl solution electrolyte bridge. The KCl electrolyte bridge provides contact with the test solution at high temperature and pressure inside the autoclave system. Usually, the cooled salt bridge and the external reference electrode operate at the same pressure as the high temperature vessel.
Many different electroactive reference elements such as Ag/AgCl, Hg/Hg.sub.2 Cl.sub.2 and Cu/saturated CuSO.sub.4 can be employed in the inner compartment of the pressure balanced system. For most applications, the Ag/AgCl electrode has been preferred. The Ag/AgCl half-cell is established when a Ag/AgCl element is immersed in a chloride solution. This results in the equilibrium EQU Ag+Cl.sup.- =AgCl+e.sup.- E.degree.=-0.222 V (1)
In a 0.1N KCl solution at 25.degree. C., the potential is 288 mV vs. the standard hydrogen electrode. The use of electro-chemical sensors for the monitoring of electrode potentials of metallic components in high temperature-high pressure aqueous environments is still not a straightforward technique, although research into the establishment of suitable electrochemical corrosion testing methods for application in high temperature pressurized systems is already 15 to 20 years old. The experimental difficulties are basically related to the combined presence of high temperature and high pressure, which produce leaks. Electrodes are normally constructed with glass or Teflon. However, electrochemical potential measurements at temperatures greater than 290.degree. C. are difficult because glass reacts with high temperature water and Teflon looses its structural strength. For temperatures greater than 120.degree. C. glass melts. For temperatures greater than 290.degree. C. the creep resistance of Teflon is so low that it has no structural integrity.
Other methods to take measurements in high temperatures have also been tried. U.S. Pat. No. 4,725,399 to McCulloch et al. discloses a probe for measuring heat which includes an elongated rod fitted within a sheath, and a plurality of annular recesses formed on the surface of the rod in a spaced-apart relationship to form annular chambers that are resistant to heat flow.
U.S. Pat. No. 4,636,292 discloses an electrode for electrochemical measurements in aqueous solutions at high temperature with a casing comprising sintered particles of aluminum oxide, zirconium oxide or other electrically-insulating material which is inert to water.
Stanford Research International (SRI) has developed an external pressure balanced Ag/AgCl electrode that does not use Teflon in the high temperature section of the electrode. The electrode has been successfully used to temperatures up to 340.degree. C. A porous zirconia plug is attached to the ceramic zirconia tube by placing the plug inside a zirconia tube that has a hole in the distal end of the tube in the direction of the tube axis of rotation. The plug is pushed to the closed end. The tube is then packed with coarse zirconia sand soaked in KCl. Another zirconia plug is placed in the open end of the tube to retain the sand. The zirconia sand section represents the "outer" high temperature electrode chamber. The internal zirconia plug separates the "outer" chamber from the "inner" low temperature electrode chamber. The "inner" low temperature chamber is filled with KCl solution. Normally, 0.1N or 0.01N KCl solution is used. Chlorine contamination of the test solution is minimized by using a porous plug. The fine porosity of the porous plug allows electrical conductivity to be established between the Ag/AgCl element and the test sample. Further, the bottom porous plug restricts chloride transport from the electrode into the test solution. In the case of high temperature water, zirconia plugs are used because zirconia is insoluble.
The SRI electrode has four disadvantages. The first disadvantage is that the electrode is very delicate. The high temperature zirconia ceramic tube easily fractures. Zirconia tube fractures have been observed to occur due to handling or thermal shock. Heatup thermal shock occurs whenever the heatup rates are greater than about 100.degree. F./h. Cooldown thermal shock occurs whenever the test system pressure or temperature limit switches trip and the autoclave naturally air cools down from operating temperatures greater than 290.degree. C.
The second disadvantage is that the zirconia ceramic tube of the SRI electrode can develop fine porosity holes with extended use in high temperature and high pressure water environments. Electrodes fabricated with CaO stabilized zirconia tubes exhibited porosity in the region of the tube exposed to the high temperature-high pressure PWRRCS primary water (i.e., distilled water with impurity additions of boron and lithium). Zirconia ceramic tubes are typically fabricated using stabilization impurities. Examples of these impurity additions are CaO, MgO and Y.sub.2 O.sub.3. The impurities are typically in the range of 3% to 12%. Note that these impurities may be associated with second phases that are susceptible to primary water corrosion. The CaO stabilized zirconia tubes exhibited through wall porosity after 1.5 to 2 months of exposure. Similar behavior is expected with Y.sub.2 O.sub.3 stabilized zirconia. Autoclave tests with Y.sub.2 O.sub.3 stabilized zirconia samples exhibited weight loss. This demonstrates that the material is dissolving in the water solution.
The third disadvantage is the chloride contamination of the test solution. Zirconia sand is used in the high temperature KCl bridge to retain the zirconia porous plug. The maximum packing density of the sand is about 65%. When the electrode is heated to the test temperature the KCI thermally expands and is forced through the zirconia porous plug into the test solution. In the case of primary water testing, chloride contamination is undesirable. One embodiment of the invention reduces this contamination by a factor of 5 relative to the SRI electrode.
The fourth disadvantage of the SRI electrode is that a post test room temperature calibration cannot be performed, when the test temperature is above about 325.degree. C. After cooldown, the inner low temperature chamber usually contains a large volume of gas and a small volume of liquid KCl. The gas is attributed to the zirconia sand and the decrease in gas solubility at intermediate temperatures. The zirconia sand has a large surface area which could absorb a large quantity of dissolved gas at high temperature. Evidently, when the electrode is cooled down from the test temperature the gas is released and exceeds the solution solubility limit. At room temperature, the gas disrupts electrical continuity and a room temperature calibration to see how the electrode potential has changed cannot be performed.
A novel improved electrode is needed which incorporates rugged high temperature-high pressure electrode components and preferably eliminates the zirconia sand.